I've recently noticed a problem that sometimes occures when we are using OpenEMM.
The creation of the mailing goes without a problem, we send the mailing, no errors in the GUI.
But after few minutes and few thousands of email sent, the sending is terminated with the following error in the xmlback log:

The file "df000B32000000039A0" exists in the OpenEMM var/spool/QUEUE directory and remains there.

1. Do you know what could be the cause of tihs error?
2. Can the sending of this mailing be resumed? I've found the .SYNC file in the OpenEMM var/spool/META directory containing the list of all the customers that the mailing was sent to successfuly.
3. Are the xmlback errors visible somewhere in the GUI?

thank you for your reply. You were right, server ran out of disk space for the mail queue.
Our monitoring system did not raise any alarms because the disk space consumption spiked, it was reduced to normal levels within the check interval.
After the process that was generating generating new emails terminated because it ran out of disk space, sendmail delivered queued email really quickly. When inspecting the server after the incident, the disk was empty so it was not obvious at first what caused the problem.